The present invention relates to intravascular imaging catheters. In particular, intravascular catheters, such as intravascular ultrasound (IVUS) catheters, which utilize a transducer mounted on a rotating core within a stationary sheath.
Ultrasonic catheter imaging systems have been developed to provide cross-sectional structural images of blood vessels, normally coronary arteries. For example, a fluid filled sheath surrounding an ultrasonic transducer subassembly at the distal end of an imaging core may be used to accomplish such imaging. The sheath is positioned within a blood vessel at the site of interest (i.e. the location of a stenosis). The transducer generates a series of sonic pulses which are transmitted outward from the transducer subassembly as it is rotated. Echo pulses reflected from the surrounding tissues are received by the transducer subassembly and collected by a control apparatus coupled to the proximal end of the sheath. The collected data is then displayed as a cross-sectional ultrasonic image of the vessel and surrounding tissue.
A transducer subassembly is generally rotated by a control apparatus at an approximate rate of 1800 Revolutions Per Minute (RPM) within the sheath. Like sonar, the transducer, periodically emits a sonic pulse through the sheath. Echoes of the transmitted pulse are returned from the surrounding tissues (including the blood vessel wall of interest). The echoes travel through the fluid filled sheath to the transducer subassembly and are eventually collected by the control apparatus which forms the ultrasonic image of the blood vessel and surrounding tissue.
A transducer subassembly is located within the apparatus near the distal end of a long thin imaging core assembly. The assembly may be made of a duplex spring originating proximally at a control apparatus and surrounding a coaxial cable. The core provides communication between the control apparatus and the transducer subassembly. The control apparatus rotates the proximal end of the core at a constant rate, typically 1800 RPM. The control apparatus excites the transducer subassembly to generate an ultrasonic pulse at equal intervals of rotation, typically every 1.5xc2x0 or less. Thus, approximately 240 or more cycles of image data (i.e. transmissions and echoes) are generated and collected by the control apparatus as the proximal end of the core makes each 360xc2x0 rotation. This equates to image data collection occurring approximately every 0.14 milliseconds. The collected data is used to create the cross-sectional image of the vessel and surrounding tissue.
While the control apparatus directs a constant rate of rotation at the proximal end of the core, the distal end of the core is only guaranteed to rotate at an average rate equivalent to that of the proximal end of the core. The rotation rate of the distal end will not be constant. Rather, the rate will fluctuate within each rotation. This varying rotational rate of the distal end of the core is due to flexural and dimensional non-uniformities of the core. The rotation rate of the distal end of the core will both increase and decrease within each rotation. This effect is generally referred to as xe2x80x9cwhippingxe2x80x9d. As described earlier, the transducer is located at the distal end of the core. Therefore, the transducer""s rotation rate within each rotation is variable in the same manner as the distal end of the core.
When the transducer is rotating at a rate less than what has been directed by the control apparatus due to whipping, sonic transmissions will occur more frequently within a given area than what has been directed by the control apparatus. That is, more than one transmission occurs per every 1.5xc2x0 of transducer rotation. This results in that sector of the image displayed by the control apparatus being expanded. Likewise, when the transducer subassembly is rotating at a rate greater than what has been directed by the control apparatus, the corresponding sector of the displayed image will be compressed due to a less than accounted for transmission per degree rate. The end result is that an image is displayed having a fairly consistent but distorted image. While the image is fairly consistent due to rotational consistency, which is discussed later, it is nevertheless inaccurate and not a true depiction of the vasculature. This distortion of the angular mapping of the collected data is referred to as Non-Uniform Rotational Distortion (NURD).
Other intravascular imaging techniques such as Optical Coherence Tomography (OCT), are also susceptible to NURD. In OCT systems, image depth scanning is accomplished by varying the path length of a reference light beam. More image data collections per core rotation may be performed by an OCT than with a sonic imaging system. In spite of these characteristics, OCT is nevertheless susceptible to NURD.
Structurally, it is difficult and expensive to reliably manufacture a low NURD core. Therefore, attempts to correct or compensate for NURD in alternative manners have been made. For the most part, these attempts have involved the use of reflectors disposed within the sheath of the imaging catheter or spline shaped sheath cross-sections. In theory, the transducer would detect the reflectors or spline shapes of the sheath, and thereby be able to correct for NURD. However, in both cases, the resulting image is distorted. Now, instead of NURD, the images have been distorted by irregularities of the sheath wall or reflectors disposed within the sheath. NURD has been exchanged for an obstructed view which is only alleviated by the reduction in reflectors or the irregularity of sheath wall shape. Furthermore, in the case of irregularly shaped sheath walls, effective extrusion is very difficult, if not impossible, to achieve.
Therefore, a need exists in the art for an apparatus and method in which the NURD resulting from the whipping of rotating core assemblies can be significantly reduced if not eliminated altogether. It is desirable that this need be met in a manner which does not result in alternative distortions.
The present invention provides a method of determining the angular position of a sensor within a catheter having a sheath wall of varying thickness. The sensor is rotated and a center frequency emitted toward the sheath wall in order to obtain echoes which may be converted into thickness data to establish the angular position. A method of imaging is also provided.
An apparatus is provided with a rotatable core having a sensor. The core is located within a sheath having a wall of varying thickness.